Spark Plugs and Ignition Performance

Copyright AA1Car Adapted from an article written by Larry Carley for Import Car magazine

Platinum and iridium have become a buzzword for ignition durability. Platinum and iridium are good conductors of heat and electricity. Both metals also have the ability to resist chemical corrosion and electrical erosion. That is why
platinum and iridium are used on the electrodes of many spark plugs today.

Some plugs have a solid platinum or iridium center electrode, while others have a
small button of platinum welded onto the tip of center electrode or both
electrodes (single platinum vs. double platinum). Some even use an alloy of platinum and iridium in the center electrode (Bosch Platinum IR Fusion).

Platinum and iridium are used because they minimize electrode wear. Every time a plug
fires, a tiny amount of metal is vaporized and erodes from the surface of
both electrodes. The center electrode typically suffers the most wear
because it runs hotter than the side electrode.

As the electrodes wear, the air gap across which the spark must jump
becomes wider and wider. The gap on a standard spark plug grows about
0.00063 to 0.000126 inch for every 1,000 miles of normal driving. And the
wider the gap, the greater the voltage needed to jump the gap.

On standard plugs with conventional electrodes, the firing voltage
requirements typically creep up about 500 volts for every 10,000 to 15,000
miles of driving. Eventually, the plugs may need more volts to fire than
the coil(s) can produce, resulting in ignition misfire. On OBD II-equipped
vehicles, too many misfires will cause the Check Engine light to come
on.

Using platinum or iridium almost eliminates electrode wear. Platinum and iridium are both expensive metals,
but they can double or even triple a spark plug's normal service life, from
30,000 to 45,000 miles for a standard plug, up to 100,000 miles
or more. Most aftermarket spark plug suppliers do not make
specific mileage claims for their platinum or iridium spark plugs, but say to follow the
OEM replacement intervals which, in most cases, is 100,000 miles.

Though long-life platinum and iridium spark plugs cost more than standard spark plugs,
OEMs use them because they reduce the risk of misfire (which helps protect
the catalytic converter) and they reduce the need for maintenance, which
allows them to offer 100,000-mile "tuneup" intervals.

One important point to keep in mind about platinum plugs is that all
platinum plugs are not the same. There are differences in electrode
configurations, design and durability. Some provide better fouling
resistance than others, and some are not recommended for use in
turbocharged or supercharged engines.

BEYOND PLATINUMPlug manufacturers have also using other exotic
materials in spark plug electrodes to extend plug life. Bosch uses a
nickel-yttrium alloy for the side electrodes in its "Platinum+4" and
"Platinum2" spark plugs. In Europe, Bosch has a spark plug that uses yttrium
for both the center and ground electrodes. For years, Champion
manufactured a premium plug with a gold-palladium center electrode and
copper-filled side electrode. That plug has now been replaced by one with
a platinum-tipped center electrode. Autolite uses a chromium-nickel alloy
for the ground electrode with its platinum-tipped center electrode plug,
while ACDelco uses a silver-nickel alloy side electrode with its
platinum-tipped plugs. NGK and Denso have premium spark
plugs with iridium alloy electrodes.

IRIDIUM SPARK PLUGS

Iridium spark plugs are now replacing platinum spark plugs in many OEM engines, not just Japanese engines. Platinum plugs are great, but are also expensive compared to iridium. So iridium plugs are now being used for more long-life (100,000 mile) applications.

Iridium plugs typically have a fine wire center electrode that forms a spark easily and reduces misfires. Iridium actually has a higher melting point than platinum, and is six times harder and eight times stronger,making it very wear resistant. The small fine wire (0.4 to 0.7 mm) center electrode concentrates the spark as well as heat. This reduces the voltage required to create a spark and fire the plug. Because of this, and the high wear resistance of the metal, iridium plugs are often recommended for "waste spark" distributorless ignition systems, and coil-on-plug ignition systems.

Familiar brands of iridium plugs include NGK Iridium and Iridium IX, and Denso Iridium Power. These are now joined by AC Delco Professional Iridium, Autolite XP Iridium, Champion Iridium, and Bosch IR Fusion (uses a platinum-iridium alloy in the center electrode). In June 2011, Bosch introduced a new line of Iridium spark plugs that uses iridium only for the center electrode.

SPARK PLUG FOULING RESISTANCE

Regardless of the type of alloy used in the
electrodes, all spark plugs must be able to resist fouling. The trick here
is to design the plug so that the electrodes run hot enough to burn off
any deposits but not so hot that they cause preignition or detonation. To
burn off carbon deposits, the electrodes need to reach about 700 degrees F
quickly. But if the electrodes get too hot (above 1,500 degres F), they can
ignite the fuel before the spark occurs, causing preignition and
detonation. For most plugs, the ideal operating temperature is around
1200 degrees F.

The temperature of the electrodes is controlled by the length of the
ceramic insulator that surrounds the center electrode and the design of
the electrode itself. Ceramics do not conduct heat very well, so an
insulator with a relatively long nose will conduct heat away from the
electrode more slowly than one with a relatively short nose. The longer
the path between the electrode and the surrounding plug shell, the slower
the rate of cooling and the hotter the plug.

Many plugs have a copper core center electrode. Copper is an excellent
conductor of heat, and allows the plug to dissipate heat quickly under
load yet remain hot enough at low speed and idle to burn off fouling
deposits.

A spark plug's "heat range" (heat rating) therefore depends on the
length of the ceramic insulator and the design of the center electrode.
The heat range must be carefully matched to the engine application,
otherwise the plugs may experience fouling problems or run too hot and
cause preignition/detonation problems. Most plugs today have a relatively
broad heat range, which means they reach a self-cleaning temperature
quickly but do not get too hot under load. This allows plug manufacturers
to consolidate applications and use fewer plugs to cover a wider range of
engines.

When replacing spark plugs, the heat range must be correct for the
engine application. Always follow the vehicle or spark plug manufacturers
recommendations. If the plugs are too cold, fouling may occur if the
vehicle spends a lot of time idling or is used only for short trips
(especially during cold weather). If the plugs are too hot, the engine may
experience preignition and detonation under load or during hot
weather.

In some situations, a slightly hotter or colder plug may be installed
than the one normally recommended. Switching to a slightly hotter plug can
help reduce fouling in an older engine that uses oil. A hotter plug can
also reduce fouling in vehicles that spend a lot of time idling or are
used only for short-trip, stop-and-go city driving. But a hotter plug
should not be used unless an engine is experiencing a fouling problem
because of the increased risk of preignition and detonation. Switching to
a slightly colder plug can reduce the risk of preignition and detonation
in performance applications (especially turbocharged and supercharged
engines), in vehicles used for towing or in those that are driven
primarily on the highway.

SPARK PLUG ELECTRODE DESIGNS

V-notched center electrodes, etc. Though each plug manufacturer takes a
slightly different approach and claims various benefits for their design,
the basic idea is to make it as easy as possible for the spark to jump the
gap and ignite the fuel mixture. A spark jumps more easily from a sharp
edge than a rounded blunt edge. That is one reason why new plugs require
less firing voltage than old ones with worn electrodes.

The electrodes on some spark plugs are also designed to "unshroud" the
spark for easier ignition. This allows the flame kernel to expand more
rapidly and reduces the quenching effect that could cause a misfire.
SplitFires V-shaped ground electrode as well as Boschs surface gap
Platinum+4 and Platinum2 plugs are all designs that claim to expose more
of the spark to the fuel mixture.

SPARK PLUG REPLACEMENT TIPS

Spark plugs should be replaced at
the vehicle manufacturers recommended intervals, but may have to be
replaced sooner if they are fouled. On older vehicles, the replacement
interval is typically 30,000 to 45,000 miles. On newer vehicles with
platinum plugs, it may be as high as 100,000 miles. But short trip
stop-and-go city driving as well as extended idling can shorten the life
of any spark plug.

Most spark plugs are pregapped. Even so, the gap may have to be reset
for some engines because of consolidations. One exception is Bosch
Platinum+4 and Platinum2 spark plugs, which are all gapped the same (1.6
mm) and must not be changed from the original setting.

Caution: Wait until the engine has cooled to replace the spark plugs if your engine has an aluminum cylinder head. If you try to remove the spark plugs while the engine is hot, there is a much greater risk of damaging the spark plug hole threads in the head.

SPARK PLUG WIRES

Good plug wires are just as important to ignition
performance as the spark plugs. Three basic types of wires may be
used:

* Distributed Resistance wire. This type of wire has a fiberglass
core impregnated with latex graphite. It provides the maximum amount of
radio frequency interference (RFI) suppression. RFI occurs when high
voltage passes through the plug wires. Creating a controlled amount of
resistance in the wire (3,000 to 12,000 ohms per foot) suppresses RFI and
prevents sensitive onboard electronics from picking up false signals that
could cause driveability problems.

One of the drawbacks of carbon core suppression wires is that internal
resistance creates internal heat. Over time, this ages the carbon core,
causing resistance to increase. And, as resistance goes up, so does the
chance for misfire.

Prior to 1980, 95 percent of all vehicles were equipped with carbon
core suppression wires. But concerns over emissions and long-term
reliability led many of the Japanese OEMs to switch to "mag"-style spark
plug wires.

* Inductance (mag) wire. This type of wire has a spiral wound core
of copper/nickel alloy wire. RFI is suppressed primarily by the magnetic
field formed by the loops of wire wrapped around the core rather than the
resistance of the wire itself. Mag wire has less total resistance (only
about 500 ohms/foot) than suppression wire, so it reduces the current
needed to fire the plugs. But its main advantage is improved durability
over the long run.

Mag-style spark plug wires have been used on Honda and Acura engines
since 1971, most Nissan and Infiniti applications since 1980, and many
Toyota and Lexus applications since 1984.

* Fixed Resistor wire. This type of wire has a steel or copper
metallic core with a fixed resistor in the plug boot to control RFI. This
wire is used on many European imports.

SPARK PLUG WIRE INSULATIONThe type of insulation used in a plug wire is also
important for long-term durability and performance. Premium wire sets
typically use silicone or EPDM (Ethylene Propylene Diene Monomer). One
thing to keep in mind about silicone is that there are different grades of
silicone. The "good stuff" can withstand temperatures from

-90 degrees F up to 600 degrees F. But according to one manufacturer, some so-called
silicone wire sets contain only 3 percent silicone, which does not provide
the same degree of temperature and chemical resistance as pure silicone.
Other premium wire sets may add a protective outer covering of EVA
(Ethylene Vinyl Acetate) for added temperature resistance and tensile
strength.

Under the tough outer silicone jacket is fiberglass braiding for
strength and flexibility, and under that is a layer of EPDM insulation
that prevents arcing and voltage leaks. Mag core wire is surrounded by a
latex silicone bonding layer that provides additional stability and
support to hold the wire in place.

DIAGNOSING PLUG WIRES

Plug wires should always be inspected when
the spark plugs are changed and any time there is a misfire complaint.
Start with a visual inspection for obvious damage such as burned or
cracked insulation, chaffing, contact with the exhaust manifold, loose
plug boots or terminals, etc. Any wires that are burned or damaged must be
replaced. The same goes for wires with loose or damaged boots or
terminals.

Next, start the engine, then look and listen for arcing while the
engine idles. A snapping or cracking noise would tell you secondary
voltage is finding a shortcut to ground. Observing the engine in the dark
may help you see where the voltage is leaking. Any fireworks that are
visible along the length of the cables or at the ends would tell you new
wires are needed.

Still can't find a bad wire? Look at the secondary firing pattern on an
oscilloscope. A bad plug wire with excessive internal resistance may cause
an intermittent or steady misfire that is usually most noticeable under
load. This will cause an increase in the affected cylinder's firing
voltage. An open plug wire or spark plug will cause the firing voltage for
that cylinder to spike to the coils maximum output.

If you find that the firing voltage is high in a cylinder, turn the
engine off and measure the plug wire's resistance end to end with an
ohmmeter. Refer to the manufacturers specifications. If resistance
exceeds specifications, the wire needs to be replaced.

A shorted ignition cable or grounded spark plug will cause a drop in
the firing voltage. Rubbing a grounded probe along the length of each plug
wire while the engine is idling may help you find any weak spots in the
insulation.

When one cylinder in the superimposed display has a firing line higher
than the rest and a shorter spark duration, high secondary resistance is
indicated. High secondary resistance may be caused by bad plug wires or
worn spark plugs, but also a lean fuel condition.

To further isolate the cause, the KV demand for the affected cylinder
should be compared to the other cylinders. If the required firing voltage
is 20 percent or higher than the rest, the problem is either too wide a
plug gap or a lean fuel condition. But if the firing voltage is roughly
the same as the other cylinders, the likely cause is high resistance in
the plug wire or spark plug.

REPLACING PLUG WIRES

If more than one plug wire has excessive resistance, replace the
entire set.

Handle cables with care during installation. Do not jerk, force,
twist or bend sharply.

Start with the longest or shortest wire first, and change one wire
at a time to avoid mixing up the firing order.

Listen for the "click" to make sure the plug boots and end terminals
are fully seated.

Follow the original cable routing to avoid crossfire problems.
Cables for cylinders that fire consecutively should not be routed
parallel or be in close proximity to one another. Keep them separated by
several inches or cross them.

All cables should be supported by wire looms, and kept away from
exhaust manifolds and sharp edges.

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